1. Technical Field
The invention pertains to a control circuit for a radiation source comprising
means for generating an error signal which is indicative for a difference between a measured average value of the output power of the radiation source and a desired average value of the output power of the radiation source,
combining means for generating a control signal for the radiation source in response to said error signal and to an information signal for modulating the radiation source.
The invention also pertains to an apparatus for reading and/or writing a record carrier comprising the control circuit.
The invention further pertains to an optical transmitter comprising the control circuit.
2. Related Art
The invention further pertains to a method for controlling a radiation source.
From U.S. Pat. No. 4,796,267 a laser controller is known which comprises a negative feedback loop to control an average light quantity level despite variations in temperature. The feedback loop comprises a sensor for generating an output signal which is a measure for the power of radiation of the radiation source, a low-pass filter for low pass filtering the output signal. The feedback loop further comprises a subtractor for generating a difference signal which is a difference between said low-pass filtered signal and a setpoint signal representing the desired average light quantity level. The known circuit maintains the average light quantity level when the temperature changes. However if the temperature changes also the proportionality factor between the control current and the light quantity level of the semiconductor radiation source changes. This implies that the difference between the output levels of the radiation source changes with a changing temperature.
Also control circuits are known which include feedback means which monitor the respective power of the radiation for each modulation level and adapt the control current accordingly. This has the disadvantage that the feedback means require a large bandwidth.
It is a purpose of the invention to provide a control circuit for a radiation source which is capable of maintaining a plurality of intensity levels at a predetermined value, using a feedback loop with a relatively low bandwidth. It is a further purpose to provide an apparatus for reading and/or writing a record carrier comprising such a control circuit, and an optical transmitter comprising such a control circuit. It is also a purpose to provide a method for controlling a radiation source having this capability. According to the invention the control means comprise
first means for modifying the information signal by a multiplicative factor xcex3 which is dependent on the error signal, and
second means for modifying the information signal by an additive factor "sgr" which is dependent on the error signal.
The output power of a laser diode is dependent on temperature (T). The output power (P) may be approximated by
P=0 for I less than Itxe2x80x83xe2x80x83(1a)
P=xcex7(T)*(Ixe2x88x92It(T)) for I greater than =I1xe2x80x83xe2x80x83(1b)
wherein Itis a threshold current required for activating the laserdiode and xcex7 is a slope (relative efficiency). Both the threshold current It and the slope xcex7 are dependent on the temperature. In the control circuit of the invention the information signal is modified by a multiplicative factor xcex3 and an additive factor "sgr" and both factors are dependent on the error signal. This enables the control circuit to perform each of the different functions such as reading, writing, erasing and preheating at the appropriate power levels despite variations in temperature.
It has been found that for most semiconductor light sources the following relationship exists between the threshold current and the slope:                                           η            0                    η                =                              (                          1              -              a                        )                    +                      a            ⁢                                          I                th                                            I                th0                                                                        (        2        )            
Therein xcex7, xcex70 are the slopes at a temperature and a reference temperature respectively, while Ith, Ith0 are the threshold currents at those temperatures and a is a dimensionless constant which is characteristic for each semiconductor light source. The value of this constant usually is within the range of 0.5 to 1.5. Hence in order to compensate at the same time for both the influence of the temperature on the slope and on the required threshold current, in a practical embodiment the multiplicative factor xcex3 and an additive factor "sgr" are related according to the function
xcex3=k"sgr"+b,xe2x80x83xe2x80x83(3)
wherein                     k        =                              a            ⁢                                          γ                0                                            σ                0                                      ⁢                          xe2x80x83                        ⁢            and            ⁢                                          xe2x80x83                            ⁢                              xe2x80x83                                      ⁢            b                    =                                    γ              0                        .                                              (        4        )            
Therein xcex30 and "sgr"0 respectively are the values of xcex3 and "sgr" at a reference temperature.
A control circuit according to the invention can be implemented in different ways. In a first preferred embodiment the additive factor "sgr" is linearly dependent on the error signal Perr, while the multiplicative factor xcex3 is computed from "sgr" according to the relation 3 above. Such an embodiment of a control circuit according to the invention is characterized in that the first means comprise modification means which calculate a modified error signal from the error signal and multiplication means for generating an intermediate signal which is indicative for the product of the modified error signal and the information signal and in that the second means comprise addition means for generating an output signal which is indicative for the sum of the intermediate signal and the error signal, the output signal being the control signal.
A variant of this preferred embodiment is characterized in that the first means comprise modification means which calculate a modified error signal from the error signal and multiplication means for generating an intermediate signal which is indicative for the product of the modified error signal and the information signal and in that the second means comprise further multiplication means for generating a further intermediate signal which is indicative for a product of the error signal and a further information signal and addition means for generating an output signal which is indicative for the sum of the intermediate signal and the further intermediate signal, the output signal being the control signal. This embodiment provides for an additional degree of freedom for controlling the radiation source.
Likewise a second preferred embodiment is possible in which the multiplicative factor xcex3 is linearly dependent on the error signal. In that case the additive factor "sgr" can be computed from the multiplicative factor xcex3 by the inverse of relation 3. Hence                     σ        =                              1            k                    ⁢                      (                          γ              -              b                        )                                              (        5        )            
Such a second preferred control circuit according to the invention is therefore characterized in that the first means comprise multiplication means for generating an intermediate signal which is indicative for the product of the error signal and the information signal and in that the second means comprise modification means which calculate a modified error signal from the error signal and addition means for generating an output signal which is indicative for the sum of the intermediate signal and the modified error signal, the output signal being the control signal. Various ways of feedback control are possible such that the multiplicative and the additive component comply with the mutual relation described above. However, in the first and the second preferred embodiment described above one of these two components is directly dependent on the error signal, so that the control means can be implemented in a relatively simple way, e.g. with a relatively small amount of hardware.
A preferred embodiment of the first embodiment of the control circuit according to the invention is further characterized by a node for receiving a signal which is indicative for a measured reflection coefficient of an object which is radiated by the radiation source, by subtraction means for generating a difference signal which is indicative for a difference between a desired reflection coefficient and the measured reflection coefficient, by multiplication means for generating an intermediate signal which is indicative for a product of the difference signal and a signal indicative for a desired power, and addition means for generating a signal which is indicative for the sum of the intermediate signal and a further signal indicative for a desired power.
In practice dust, fingerprints etc. may absorb a part of the radiation from the radiation source, and therewith influence its effect on the record carrier. In this preferred embodiment these influences are compensated for, while at the same time still a good compensation for temperature influences is obtained.
Preferably this embodiment is characterized by multiplication means for generating a product signal which is indicative of a product of the difference signal and the error signal and means for generating an output signal which is indicative for the sum of the product signal and the difference signal. In this way a short response time is obtained.
The controller according to the invention is in particular suitable for application in a recording/playback device using a radiation source. In accordance with the invention therefore a device for recording and/or playback of information on a record carrier includes
a controller according to the invention,
a radiation source which is controllable by the controller,
a sensor for generating a signal which is indicative of an intensity of radiation emitted by the radiation source and which is coupled to an input of the controller,
means for mapping the radiation beam at a spot at the information carrier,
means for causing a relative displacement between the spot and the record carrier.
The control circuit according to the invention is also very suitable for application in an optical transmitter. An optical transmitter according to the invention therefore comprises:
input means for receiving an input signal,
a control circuit according to one of the claims 1 to 7 coupled to said input means,
a radiation source controlled by the control circuit
measuring means for measuring an amount of radiation generated by the radiation source having an output coupled to the control circuit,
means to couple the output of the radiation source into a light conductor.
The invention also pertains to a method for controlling a radiation source comprising
determining an error signal which is indicative for a difference between a measured average value of the output power of the radiation source and a desired average value of the output power of the radiation source,
generating a control signal for the radiation source by combining said error signal and an information signal representative of information to be written to the radiation source, characterized in that, the step of generating the control signal comprises
modifying the information signal by a multiplicative factor xcex3 which is dependent on the error signal, and
modifying the information signal by an additive factor "sgr" which is dependent on the error signal.